Conventionally, a weft knitting machine is provided with a yarn feeder mounted, for example, on a side cover on one end side in a longitudinal direction of a needle bed so that a knitting yarn is fed from a yarn feeding port of a yarn feeding member to a knitting needle when knitting a knitted fabric. The yarn feeder comprises a buffer rod to have a function of temporally storing a knitting yarn, and a function of applying a tension to the knitting yarn. However, in a conventional yarn feeder with which the knitting yarn is stored and the tension is applied to the knitting yarn within a range of inclination of the buffer rod, a fluctuation in tension of the knitting yarn during knitting is increased.
There has been known a conventional technique in which a tension is applied to a knitting yarn by using a member corresponding to the buffer rod in the conventional yarn feeder, and a margin is allowed for to cope with a sudden fluctuation in tension of the knitting yarn, to thereby suppress the fluctuation in tension of the knitting yarn while actively feeding out the knitting yarn (refer to Japanese Examined Patent Publication JP-B2 2541574, for example). In addition, there has been known a conventional technique in which rotation of a spinning wheel for feeding a knitting yarn is controlled prior to a sudden change in demand for the knitting yarn, to thereby suppress a fluctuation in yarn tension without using a member corresponding to the buffer rod (refer to Japanese Unexamined Patent Publication JP-A 11-500500 (1999), for example).
The applicant discloses a technique in which a knitting yarn necessary for knitting is fed while consumption of the knitting yarn is calculated based on knitting data of a knitted fabric (refer to WO 2004/009894, for example). In WO 2004/009894, a yarn feeder 16 as shown in FIG. 8 is used. The yarn feeder 16 includes a main roller 20, a sub-roller 21, and a servo motor 22. The main roller 20 is mounted on a rotating shaft of the servo motor 22, and rotating force of the servo motor 22 is transmitted to the sub-roller 21 through a driven mechanism which is constituted by combining a plurality of gears. The main roller 20 and the sub-roller 21 are arranged so as to nip a knitting yarn 4, and the sub-roller 21 is rotationally driven at a circumferential speed equal to that of the main roller 20.
The knitting yarn 4 is fed from above a frame of the weft knitting machine, and guided to a portion in which the sub-roller 21 is opposed to the main roller 20 while keeping in contact with an outer circumferential surface of the main roller 20. A small clearance is formed between the outer circumferential surface of the main roller 20 and that of the sub-roller 21, through which the knitting yarn 4 passes. The knitting yarn 4 is further guided to an intermediate roller 25, and is oriented to a different direction and pulled to a tip side 9 of the buffer rod 7. A spring which energizes so as to raise the tip side 9 is provided on a base end side 8 of the buffer rod 7. An inclination angle of the buffer rod 7 is defined such that a rod angle becomes a zero degree at a position in which a distance between the tip side 9 and the intermediate roller 25 becomes smallest. The spring swingingly displaces the buffer rod 7 such that the inclination angle becomes small when the tension of the knitting yarn 4 is large, and the inclination angle becomes large when the tension thereof is small. The inclination angle of the buffer rod 7 is detected by an inclination angle sensor 27 which is provided on the base end side 8. The inclination angle of the buffer rod 7 can be changed, for example, in a range of 0 degree to 100 degrees.
FIGS. 9A and 9B show a necessary yarn feeding mode of the yarn feeder 16 shown in FIG. 8. FIG. 9A shows a basic change in the inclination angle in accordance with swinging displacement of the buffer rod 7 which is plotted against time during which a yarn feeding member moves a yarn feeding position with respect to a knitted fabric. In addition, FIG. 9B shows a basic change with respect to a speed of the servo motor 22 in the yarn feeder and a speed that the knitting yarn 4 is fed out to the yarn feeding member, corresponding to movement of the yarn feeding position. The changes of FIGS. 9A and 9B correspond to a case in which one course of the knitted fabric is knitted while the yarn feeding member such as a yarn feeder entrained by a carriage is moving from a side close to the yarn feeder 16 to a side away therefrom. The yarn feeding member starts its movement by use of the carriage at a time to after a rod alignment when reversing has been carried out at a time ta. When the carriage moves at a speed of V, the servo motor 22 is controlled so that the feeding member feeds out the knitting yarn at a yarn speed of 2V which is twice as fast as a carriage speed until the yarn feeding member starts its movement at the time t0 and reaches a reference position at a knitting end of the knitted fabric at a time t1. The knitting yarn required in this 2V zone includes a portion from the tip side 9 of the buffer rod 7 to the yarn feeding member, and a portion from the yarn feeding member to a knitting needle at the knitting end. When the yarn feeding member is entrained by the carriage to move away from the tip side 9 of the buffer rod at a speed of V, it is necessary to feed the knitting yarn at a yarn speed of 2V.
In the conventional yarn feeder, as described above, while the knitted fabric is knitted in the waft knitting machine, an amount of demand for knitting yarn is significantly changed depending on a movement position of the yarn feeding member, and the yarn tension is fluctuated depending on the demand for the knitting yarn. The significant fluctuation in the yarn tension leads to a change in a knitted loop length in a knitting width direction of the knitted fabric, resulting in deterioration in quality of the knitted fabric.
Also in the conventional technique disclosed in JP-B2 2541574, it is difficult to cope with a sudden change in the amount of demand for the knitting yarn which is produced, for example, at an end of the knitted fabric. In the conventional technique disclosed in JP-A 11-500500, it is expected to cope with a sudden change in the amount of demand for the knitting yarn. However, in this conventional technique, it is difficult to stabilize the yarn tension while knitting a fabric because no buffer rod is provided. Further, a slack of the knitting yarn produced when the yarn feeding member has been moved to the yarn feeder side cannot be eliminated. Moreover, it is necessary to wind the knitting yarn around a spinning wheel, thereby increasing a size of the spinning wheel. Weft knitting machines often knit a knitted fabric using a plurality of yarns, and thus such flat knitting machines need to have a yarn feeding mechanism for each yarn. When weft knitting machines include a plurality of yarn feeders each of which uses a large spinning wheel, the flat knitting machines become large.
In a buffering mechanism such as the buffer rod 7 disclosed in WO 2004/009894, it is expected that a sudden change in an amount of demand or supply for the knitting yarn 4 is relieved by its mechanical motion. However, when a yarn speed is increased, the buffer rod 7 is suddenly moved to produce an overshoot and an undershoot due to inertia. For example, when knitting motion of the knitting needle is started, and a stitch cam acts on the knitting needle, a sudden pull-in of the yarn is started, and the knitting yarn 4 is suddenly pulled, so that suddenly the buffer rod 7 is swingingly displaced to a side decreasing a rod angle. When the buffer rod 7 starts its sudden swinging displacement, the buffer rod 7 is swingingly displaced due to the inertia so as to feed the knitting yarn 4 longer than a length with which the knitting yarn 4 is actually pulled, thereby producing an overshoot. Accordingly, a tension of the knitting yarn 4 at that time is reduced. The tension of the knitting yarn 4 is reduced, and thereby an amount of the knitting yarn 4 pulled in by the knitting needle is increased, resulting in a rough knitted loop. Further, when the knitting yarn 4 is loosened, the buffer rod 7 is swingingly displaced to a side increasing the rod angle. When the knitting yarn is suddenly loosened, for example, in a case in which the yarn feeding member starts its movement to the yarn feeder side, the buffer rod 7 cannot keep up with the sudden slack of the knitting yarn, thereby producing an undershoot. Accordingly, the tension of the knitting yarn 4 at that time is reduced. When knitting motion of the knitting needle is started while the undershoot is continued, an amount of the knitting yarn 4 pulled in by the knitting needle is increased, resulting in a rough knitted loop. Thereafter, the buffer rod 7 continues its swinging displacement to a side increasing the rod angle due to the inertia, producing an overshoot to such a direction that the rod angle is increased, instead of being to carry out its swinging displacement to such a direction that the rod angle is decreased in order to feed the knitting yarn 4 to the yarn feeding member side when the knitting motion is started. Accordingly, the tension of the knitting yarn 4 at that time is increased. When the tension of the knitting yarn 4 is increased, clogging that the knitted loop becomes small is produced. When the tension is excessively increased, the knitting yarn 4 may possibly be broken. Such fluctuation in the tension by an effect of the inertia of mechanical parts should be produced when it is attempted to feed the yarn at a higher speed, for example, using the spinning wheel disclosed in JP-A 11-500500.